iForest - Biogeosciences and Forestry


Essential environmental variables to include in a stratified sampling design for a national-level invasive alien tree survey

Johann DF Kotze (1-2)   , Hein B Beukes (1), Thomas Seifert (2-3)

iForest - Biogeosciences and Forestry, Volume 12, Issue 5, Pages 418-426 (2019)
doi: https://doi.org/10.3832/ifor2767-012
Published: Sep 01, 2019 - Copyright © 2019 SISEF

Research Articles

There is a direct relationship between the abundance of biological invasions and their impact, which means that it is important to capture spatial patterns in their abundance and use this information to focus management actions. However, protocols to objectively determine invasive alien plant (IAP) distributions and abundance are lacking at a national level, resulting in the inability to determine and monitor changes in spatial extent and density over time. A complete inventory of IAP spatial distribution across an extensive area such as South Africa is not possible and so requires an efficient sampling approach. A simple random sampling design would not be efficient, so monitoring of IAP species at a national level requires an appropriate sampling design such as a stratified sampling. The selection of environmental variables to be included in such a stratification should be based on the relationship between IAP species and their physical environment to successfully summarize variance in their abundance within the different strata. A further objective is to obtain all possible combinations of environmental variables or a full rank design in the stratification to allow for the comparison of different strata based on actual field sampled data. This raises the question of which predictive environmental variables as well as how many to include in the stratification. For this purpose, three invasive tree species, namely Acacia cyclops, Acacia mearnsii and Prosopis glandulosa were selected as they cover the maximum possible area at the highest density with the least amount of geographic overlap. A total of 26 environmental variables that included climatic, soil and topographic type variables were tested with linear regressions against correlations with the abundance of those tree species. The results showed that a combination of average precipitation, soil depth, clay content in the B-horizon and terrain morphological units will serve as a suitable stratification at a national level to explain IAP abundance variation sufficiently well whilst retaining a full rank design. These results will be applied as the first phase in the formation of a regional level IAP monitoring programme for South Africa on a scientific basis.


Invasive Alien Plant (IAP) Species, Monitoring, Sampling Design, Stratification, Environmental Variables

Authors’ address

Johann DF Kotze 0000-0002-9632-5904
Hein B Beukes
Institute for Soil, Climate and Water, Agricultural Research Council, Private Bag X79, Pretoria, 0001 (South Africa)
Johann DF Kotze 0000-0002-9632-5904
Thomas Seifert 0000-0002-9611-6272
Stellenbosch University, Department of Forest and Wood Science, Faculty of AgriSciences, Private Bag X1, Matieland, 7602 (South Africa)
Thomas Seifert 0000-0002-9611-6272
Chair of Forest Growth, Albert-Ludwigs-University Freiburg, Tennenbachstraße 4, 79106 Freiburg (Germany)

Corresponding author

Johann DF Kotze


Kotze JDF, Beukes HB, Seifert T (2019). Essential environmental variables to include in a stratified sampling design for a national-level invasive alien tree survey. iForest 12: 418-426. - doi: 10.3832/ifor2767-012

Academic Editor

Francisco Lloret Maya

Paper history

Received: Feb 23, 2018
Accepted: Jun 12, 2019

First online: Sep 01, 2019
Publication Date: Oct 31, 2019
Publication Time: 2.70 months

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